A group III nitride semiconductor is a semiconductor material composed of a mixed crystal of indium nitride (InN), gallium nitride (GaN) and aluminum nitride (AlN), and by controlling a composition of a mixed crystal of In, Ga and Al that are group III elements, it becomes possible to manufacture light-emitting elements having high efficiency in a wavelength range of infrared region to ultraviolet region corresponding to their respective band gap energies (0.7 eV (InN), 3.4 eV (GaN) and 6.1 eV (AlN)). On that account, blue light-emitting diodes using the group III nitride semiconductors are now being used for a wide range of applications including illumination, as white light-emitting diodes that are combinations of them and fluorescent materials.
The blue light-emitting diode is formed of an InGaN-based material that is a mixed crystal material of InN and GaN, and in general, it is manufactured by forming, on a C-plane ((0001) plane) sapphire substrate, an n-type GaN layer, an InGaN light-emitting layer and a p-type GaN layer in this order through a metal organic chemical vapor phase deposition (MOCVD) method. In this case, owing to a difference in lattice constant or thermal expansion coefficient between the sapphire substrate and the GaN layer, crystal defects (dislocation) causing lowering of luminous efficiency are highly densely formed in the GaN layer, but by the composition modulation effect of In in the active layer, high-efficiency light emission is achieved (See non patent document 1).
Since the sapphire substrate has insulation property, a structure in which the n-type and the p-type electrodes to drive the light-emitting diode are formed on the same plane side of the plane (Ga polar plane) where the group III nitride layers have been formed is generally widely adopted. Moreover, for the purpose of obtaining high output by increasing the applied current, a vertical light-emitting diode structure in which by peeling the sapphire substrate from the GaN layer through a laser lift-off method or the like or by using a conductive GaN substrate, counter electrodes have been formed on a rear plane (−C-plane, nitrogen polar plane) of the n-type GaN layer or the n-type GaN substrate and on a surface of the p-type GaN layer has been proposed (See, for example, patent documents 1 and 2).
On the other hand, in an ultraviolet light-emitting diode having a shorter wavelength than the blue light-emitting diode, an aluminum gallium nitride (AlGaN) material that is a mixed crystal-based material of GaN and AlN is used. Also in this case, a sapphire substrate is mainly used as a substrate material, and an ultraviolet light-emitting diode is manufactured by a process similar to that for the InGaN-based light-emitting element. In the case of the ultraviolet light-emitting diode, however, decrease in luminous efficiency and reliability becomes noticeable owing to the dislocation formed in the AlGaN layer, and therefore, a technique of using an AlN single crystal having a physical constant close to that of the AlGaN layer, for the substrate has been proposed. It has been reported that by adopting the AlN single crystal for the substrate, high luminous efficiency and high reliability are obtained. (See non patent document 2).
If a vertical structure can be adopted also in the ultraviolet light-emitting diode similarly to the InGaN-based light-emitting diode, much higher output can be expected. With regard to the AlGaN-based materials, however, an effective substrate peeling means, such as the aforesaid laser lift-off method, has not been developed, and in the present technology level, it is difficult to achieve a vertical ultraviolet light-emitting diode structure using the substrate peeling technique.
Another means to achieve a vertical ultraviolet light-emitting diode is a vertical ultraviolet light-emitting element structure using an n-type conductive AlN substrate (See patent document 3). In the present circumstances, however, the vertical ultraviolet light-emitting diode that uses an n-type conductive AlN substrate and emits light in the ultraviolet region has not been achieved yet.